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Slow progress: Builders are finding it hard to say not to work. Photo: Rob Homer

The average Australian detached home takes nearly twice as long to build as it did in 2000, according to new academic research that could have been ripped straight from an episode of The Block or Grand Designs.

In 2000, the average detached house - home to 78 per cent of households in 2011-12 - took only 4½ months to build. That rose to eight months in 2010, according to the most recent available data.

The Australian Housing and Urban Research Institute said one cause of the ongoing housing shortage is that homes are getting bigger, the designs are becoming grander and more complicated to build.

And builders find it hard to say no to new business, even if they know they can't finish in the time promised.

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The median floor size of new homes in growth suburbs increased 39 per cent, from 138 square metres in 1990 to 192 square metres in 2007. And not only are houses getting bigger, they are also getting far trickier to build.

''For builders this means more complex contracting arrangements to ensure the correct building materials are on-site, in the right quantities at the right time,'' said the institute's report, released on Wednesday. ''Unfortunately, more complex production systems are more prone to time-consuming errors being made, resulting in delays.''

According to Ehsan Gharaie, a lecturer at RMIT, the increased size of homes was only one factor causing delays. While new houses under construction have increased - up 55 per cent to 53,827 in September last year compared with 2001 - the average number of completed houses each year had not varied for 25 years.

''The inconsistency between the trend of number of dwellings under construction and number of completions shows that the house building industry has responded to the higher demands by signing more contracts and accepting more work. However, it could not produce higher number of houses and its output remained constant,'' Dr Gharaie said.

In other words, more buildings are being built at the same time, but each one is taking longer to complete. ''It is a structural problem in the building industry. [Builders] can't say no to new work. You just say yes, and that means the house may take longer to build … but the customer won't know.''

Dr Gharaie said this increase in house completion times was serious. In its most recent report, the National Housing Supply Council predicted the ongoing shortage in housing would reach 328,800 dwellings by 2015 and 640,200 dwellings by 2030.

In Dr Gharaie's research, he used a simulation model which estimated that it would take a builder 163 days to build a house if there were no weather delays and all tradespeople and resources are available and on-site as required. But as anyone who has watched a dramatic episode of The Block would know, one tradesperson's delays can be disastrous.

''A 70 per cent reduction in availability of just one tradesperson … could extend the building time to 527 days,'' the report said.